This invention is related to a nonwoven sheet which is useful as a primary carpet backing and, in particular, is related to said sheet having an improved selvage.
Nonwoven sheets of continuous, isotactic polypropylene filaments which are useful as primary carpet backing are known in the art. U.S. Pat. No. 3,502,538, issued to Petersen on Mar. 24, 1970, and U.S. Pat. No. 3,991,244, issued to Debbas on Nov. 9, 1976, disclose sheets which are representative of those found to be useful as primary carpet backing. The Petersen patent discloses a bonded nonwoven sheet comprising a matrix of synthetic organic fibers having a breaking strength of at least 7 grams, said fibers being interconnected at a multiplicity of points throughout the sheet, by at least some bonds having a strength greater than 0.1 gram, the average strength of these bonds being at least 0.9 gram and less than the matrix fiber breaking strength, the distribution of such bond strengths being characterized by a variance of at least 4, the number of said bonds being such that the product of the number of bonds per cubic centimeter and the average bond strength is greater than 5 .times. 10.sup.4 g/cm.sup.3 and this product divided by the fiber breaking strength is less than 9 .times. 10.sup.3 /cm.sup.3. Sheets produced from fibers of oriented isotactic polypropylene as matrix fibers and unoriented or low oriented isotactic polypropylene fibers as binder material are disclosed as preferred.
The Debbas patent discloses a length of layered, nonwoven, bonded, continuous filament, isotactic polypropylene fabric comprising a machine direction layer at each surface of the fabric with each of the two machine direction layers constituting from 20-30% of the fabric weight and a cross-machine direction layer which constitutes from 40-60% of the fabric weight, each of said layers consisting essentially of 65-90% by weight of matrix filaments and 10-35% by weight of binder, the matrix filaments in the machine direction layers having an average denier per filament of from about 6-20 and a tenacity of at least 2.0 gpd; the matrix filaments in the cross-machine direction layer having an average denier per filament of from 26-60 and a tenacity of at least 3.0 gpd, and which is at least 10% higher than the tenacity of the matrix filaments of the machine direction layers, the filaments of the layered fabric being disposed in such a manner as to provide certain specified directionality values as measured by the randometer method, said fabric having a neckdown of 1-5%.
In the production of cut-pile carpet from primary carpet backings, such as the foregoing, the backing is tufted and the resulting pile is cut. The carpet obtained thereby is beck dyed, dried on a tentering frame, sometimes latexed, may be given a secondary backing, and then dried on a tentering frame again. During the various steps of this carpet making process, fibers and groups of fibers in the non-tufted areas of the carpet backing, i.e., the selvage, become pulled away from the body of the sheet and thereby create a highly fuzzed surface which renders the carpet unsatisfactory for further processing. Furthermore, in the beck fuzzed surfaces tend to collect loose pile fibers thereby creating "fuzz balls".
U.S. Pat. No. 3,360,421, issued to Sands on Dec. 26, 1967, is directed to this fuzz problem and discloses a nonwoven sheet suitable as a carpet backing and comprising continuous filaments disposed in random fashion and bonded to one another at a multiplicity of bond points throughout the sheet, the sheet further having, at least along the edges thereof, between about 20 and 80 perforations per square inch, the filaments in the walls of each perforation being fused together throughout the depth of the perforation. The sheet is perforated in a step which is separate from the bonding operation.
In the carpet making process, it is sometimes difficult to control precisely the area of the backing which is tufted. Thus, often, due to this lack of precise control, part of one edge of the carpet backing is also tufted. It has been found that when this occurs with a nonwoven backing having a selvage stabilized as described by the Sands patent, the backing selvage suffers a significant loss in tufted tongue tear strength, thereby resulting in rupture of the sheet selvage during tentering operations. The carpet having ruptured edges is not suitable as a "first grade" material, e.g. carpet with the desired standard width, and thus is often salvaged by trimming for uses, such as throw rugs and narrow width runners.
A primary carpet backing which would provide a selvage having high fuzz and delamination resistance while maintaining a rather high tufted tongue tear strength in said selvage would offer a rather attractive advantage in that when the variability of the tufting process results in tufting of an edge of the backing, the carpet produced thereby would still be useful as a "first grade" material. Moreover, if this carpet backing could be produced during the usual nonwoven sheet production without the requirement of a separate heating operation or special heating equipment, then the commercial feasibility of producing such a selvage-stabilized backing would be greatly enhanced.